It's very good, if painful, in parts...kinda like you'd expect a scar to be, really.
It's a story about things that might be...and, of course, of scars.
The "Scar" of the title represents the destination quest ("The Scar", a hole in the world which might-have-beens puportedly spill from), but also the leaders of the city where it takes place (who practice fetechistic scarification), the sutures that hold the city Armada (made of hundreds of boats), where the story takes place, together, the scars of one of the main characters' transformation into a fish-person, the scars of the primary major character getting lashed and of growing up, the scars of the various people of Armada (many of whom are "press-ganged"—taken captive and forced to assimilate or die) joining with the city itself. And, I'm sure, there are intended scars I've missed, but the idea of a scar as a healing over a wound...or is it a wound that never heals?...pervades the book.
This combines the minor (or perhaps more major, yet less hammered apon?) theme -- of Possible Things. The most important non-protagonist character, Uther Doul, has a sword that can make every plausible strike at once, and so has trained to be able to fight such that he -could- make nearly any useful strike with his "Possible Sword" (or "Might", as in "might have been," "Sword"). The primary protagonist starts the story off with a letter to...someone; we're not told to whom, and various bits of it get written throuought the novel. Late in the story (in a nearly 600 page story, I should add, and after she's frantically tried to get it sent), she confesses that she doesn't know whom it's to either; she's left it unaddressed and figured she'd decide who it was to when it was done. At the conclusion of the story, she decides that it's a Possible Letter, capable of going in a myriad of directions and giving her (narrative, presumably) power as a result.
All together (there is -also- a plot) it's a pretty good read.